Developments in the semiconductor industry are typically governed by Moore's law which predicts that the number of transistors in a dense integrated circuit doubles every two years. As will be appreciated, this poses significant technological challenges whenever technological boundaries constituted by physical laws are encountered and need to be overcome to meet the industry demands for even smaller and smaller integrated circuits.
One typical challenge encountered already in the twentieth century, was the need to take into account interlayer misalignment between functional layers of a multilayer semiconductor device. In the multilayered structure of semiconductor devices, functional device layers are typically deposited on top of other functional device layers. The functional product features, i.e. the features of the pattern formed in each layer, need to be aligned accurately with those in contiguous functional layers to enable correct operation of the end product: the semiconductor device. This is achieved by monitoring an overlay error during manufacturing, i.e. the amount of misalignment between subsequent layers, by determining relative positions between marker elements in subsequent layers. This improvement, and the advancements achieved after this in the determination of overlay errors, has allowed to continue making semiconductor elements smaller and smaller over the past decennia, in accordance with Moore's law. The present state of the technology that is applied for measuring overlay error, is the use of diffraction optics to determine a relative distance between marker elements.
A disadvantage of the current determination methods for determining overlay error, is that the marker features that can be ‘seen’ using the diffraction optical methods applied are of much larger dimensions than the functional product features, i.e. pattern features, nowadays desired in accordance with Moore's law. Therefore, even though the correct alignment of marker elements in subsequent layers of a multilayer semiconductor device can be established by monitoring the overlay error with existing techniques, it can still not be guaranteed that also the much smaller pattern features in subsequent layers are correctly aligned. These small pattern features are differently affected by sensing errors, e.g. lens errors, of the alignment sensor used for determining the overlay error. Hence, an undesired or destructive overlay error is more and more difficult to rule out during manufacturing.